Method of producing titanium alloys having an ultrafine grain size and product produced thereby

ABSTRACT

A PROCESS IS DISCLOSED FOR PRODUCING ULTRAFINE GRAINED TITANIUM ALLOY MICROSTRUCTURES WHICH INVOLVES HEATING THE TITANIUM ALLOY BODY TO A TEMPERATURE BELOW THE ALLOY&#39;&#39;S BETA TRANUS TEMPERATURE BUT ABOVE ITS MARTENSITIC TRANSFORMATION TEMPERATURE, HOT WORKING THE HEATED ALLOY BODY AS ITS TEMPERATURE DECREASES, QUENCHING, AND REPEATING THE CYCLE AT LEAST ONCE.

3,686,041 AN ULTRAFINE REBY D. LEE

Aug. 22, 1972 METHOD OF PRODUCING TITANIUM ALLOYS HAVING GRAIN SIZE ANDPRODUCT PRODUCED THE Filed Feb. 17, 1971 3 Sheets-Sheet 1 BETA -TRA MSUSfor- 77-Mo Alloys I Hot -working range For T/'6 Mo Al/oy K fie/d r a s 5o 77-MoA//0ys l I I0 is 2'0 25 so WEIGHT Z M01. YBDE/VUM His Attorn ey.

Aug. 22, 1972 D. LEE 3,636,041

METHOD OF PRODUCING TITANIUM ALLOYS HAVING AN ULTRAFINE GRAIN SIZE ANDPRODUCT PRODUCED THEREBY Filed Feb. 17, 1971 s Sheets-Sheet a Invenfior': Daeyora Lee,

Aug. 22, 1972 LEE 3,686,041

METHOD OF PRODUCING TITANIUM ALLOYS HAVING A-N ULTRAFINE GRAIN SIZE ANDPRODUCT PRODUCEDYTHEREBY Filed Feb. l'?', 1971 5 Sheets-Sheet 3 [r7ven'or': baeyong Lee,

His Attorney.

United States Patent Int. Cl. CZZf 1/18 US. Cl. 148-115 R ClaimsABSTRACT OF THE DISCLOSURE A process is disclosed for producingultrafine grained titanium alloy microstructures which involves heatingthe titanium alloy body to a temperature below, the alloys beta transustemperature but above its martensitic transformation temperature, hotworking the heated alloy body as its temperature decreases, quenching,and repeating the cycle at least once.

This application is a continuation-in-part of my copending applicationSer. No. 787,838, now Patent No. 3,615,900, entitled: Process forProducing Articles With Apertures or Recesses of Small Cross Section andProduct Produced Thereby, filed Dec. 30, 1968 in the name of Daeyong Leeand assigned to the same assignee as the present application.

This invention relates to titanium alloy bodies having an ultrafinegrained microstructure, and to a method of producing suchmicrostructures.

Most titanium alloys cannot readily be worked at room temperature.Working and shaping of titanium alloys for high temperature use, such asfor jet engine parts, requires a fine grain size to make possible a highdegree of plastic deformation (superplasticity). In order to achieve asuperplastic behavior in such alloys, for good workability andformability it is desirable to have an ultrafine grain size in thealloy, namely about 1 to 5 microns. Such ultrafine grain size helps, notonly for high temperature working of the alloys, but also contributes toimproved mechanical properties at lower temperatures (below one-half ofthe melting point).

It is therefore an object of the present invention to provide a methodfor producing in titanium alloy bodies microstructures of ultrafinegrain size.

Another object of the invention is to provide a method for hot workingtitanium alloys which will produce therein an average grain size of lessthan about 5 microns.

Another object of the invention is to provide a method for hot working atitanium alloy body at a temperature at which the alloy is in a pluralphase condition in order to impart superplastic properties to the alloy.

Still another object of the invention is to provide titanium alloybodies having a microstructure with an ultrafine grain size.

SUMMARY OF THE INVENTION These and other objects of the invention areachieved by heating an alloy body to a temperature below the specificalloys beta transus temperature but above its martensitic transformationtemperature, hot Working the heated alloy body as its temperature cools,quenching to room temperature, and repeating the cycle at least once.

According to another feature of the invention, the specific titaniumalloy bodies contain, in their compositions, at least one beta-phasestabilizer, such as vanadium, molybdenum, iron, manganese or chromium.The hot working takes place while the alloy body is at a tem- 3,686,041Patented Aug. 22, 1972 perature at which the microstructure has aplurality of phases, at least alpha-phase plus beta-phase.

Briefly stated, the process of the present invention comprises providingan alloy having the characteristic of being comprised of at least twophases in the solid state. These two phases may be alpha phase and betaphase. The alloy is treated to produce at least one phase in anultrafine form distributed in a matrix comprised of the second or otherphases.

In a preferred embodiment of the present invention, a solidtitanium-base alloy which has a two-phase structure and which undergoespartial martensitic transformation is used. The martensitic phaseappears within a specific temperature range during cooling. The processof treating such an alloy to produce a phase in ultra-fine grain sizeform comprises providing the alloy in cast or other form, plasticallydeforming the alloy after heating the alloy to a temperature at leastabove the temperature at which the martensitic transformation occurs fora time sufiicient to homogenize the structure, quenching it to roomtemperature, re-heating the quenched solid to a temperature above thetemperature at which the martensitic transformation occurs and workingsaid hot solid to produce at least one phase in a fine form. For thistype of alloy, the working of the hot solid in the two phase solidregion, i.e. above the temperature at which the martensitictransformation occurs, results in at least one phase being produced inan ultra-fine grain size form. Repeated heating of the alloy to thetwo-phase solid region above the martensitic transformation temperature,but below the beta transus temperature and reworking of the alloy inthis region will produce a distributed phase of an even finer grain sizeform. The alloy can be hot-worked suitably by methods such as rolling orswaging.

There are a number of alloys of certain composition which are comprisedof at least two phases and which undergo partial martensitictransformation during cooling. Such alloys and their compositions areknown from the literature. Representative of such alloys are Ti-Mo,Fe-C, Ti-V, Fe-Ni, Au-Cd, Fe-Ni-C.

Generally, in carrying out the instant process, the alloy components aremelted together to obtain as uniform a molten sample as possible. Themolten sample is then cast by a conventional method to the desired size.

The cast alloy is plastically deformed to at least partially destroy itscast structure. A number of methods are suitable for carrying out suchdeformation. For example, the alloy can be worked while hot and plasticby methods such as extrusion, rolling, compression or swaging. Thespecific temperature at which the alloy is hot worked depends largely onits malleability at such temperature, but in order to obtain theultra-fine grain size of the desired phase structure the range of hotworking temperature must be between the beta transus temperature and themarten sitic temperature.

DESCRIPTION OF THE DRAWINGS The invention will be better understood fromthe following description taken in conjunction with the accompanyingdrawings wherein:

FIG. 1 is a plot of temperature versus concentration in weight percentfor molybdenum, and illustrating, as a typical example, the hot workingtemperature range of the present invention for titanium alloyscontaining, as abeta stabilizer, molybdenum;

FIG. 2 is an electron micrograph (7500X) of a titanium base alloycontaining '6 wt. percent molybdenum, and illustrating the ultrafinegrain size microstructure achieved by processing according to thepresent invention;

FIG. 3 is an electron micrograph (7500 of a titanium base alloycontaining 12 wt. percent molybdenum and processed according to thepresent invention;

FIG. 4 is an electron micrograph (7500 of a titanium alloy containing 12wt. percent molybdenum and 0.4 wt. percent silicon processed accordingto the present invention;

There are broadly two categories of alloys where ultrafine grain sizemay be obtained. In eutectic and similar alloys, the fine grain size isprovided by the inherent structure itself. However, in titanium alloys,where various forms of phase transformation takes place, ultrafine grainsize is not inherent in the structure itself and heretofore no simpleprocess was known for achieving it. The present invention accomplishesthis and by thermomechanical processing; that, is by a series of stepswhich includes hot working the alloy at a temperature below thebetatransus temperature, but above the martensitie transformationtemperature, and then quenching from the hot working temperature, andrepeating the heating and quenching.

All parts, proportions or amounts used herein are by weight unlessotherwise noted.

The invention is further illustrated by the following examples.

Example 1 A 94% titanium-6% molybdenum alloy button (Composition A ofTable I) was cast in a vacuum by means of an arc-melting. Each of thecomponents was about 99.999 percent pure. The button was about inchthick. Two opposed periphery portions of the button were machined off toproduce parallel sides. The resulting structure, i.e. workpiece, had adiameter of two inches and was about /4 inch in height. It was wrappedin titanium foil to prevent oxidation of the titanium and heated in afurnace having an atmosphere of purified helium. All subsequent beatingsof the alloy workpiece were also carried out in an atmosphere ofpurified helium. When the workpiece attained a temperature of 1200 C.,it was removed from the furnace and forged by means of a drop hammer toa thickness of 0.385 inch to destroy its cast structure.

The workpiece was then heated in the furnace to a temperature of 800 C.and was maintained at this temperature for 30 minutes to homogenize itsstructure and then water quenched to room temperature.

The workpiece was then heated to a temperature of 700 C. which is abovethe temperature at which the martensitic transformation occurs, and hotrolled for about seconds. This heating and hot rolling procedure wasrepeated two more times and then the workpiece was water quenched toroom temperature. Its thickness by this procedure was reduced to 0.243inch. It was then reheated to 750 C., hot rolled and water quenched toroom temperature resulting in a thickness of 0.187 inch. It was thenheated to 800 C., hot rolled and water quenched to room temperatureresulting in a thickness of 0.138 inch.

The workpiece was then heated to 750 C. and main- 4 tained at thistemperature for /2 hour to stabilize its structure. It was then rapidlycooled in air to room temperature.

The temperature at which the martensite transformation from beta solidsolution to the alpha prime supersaturated solid solution takes place isreferred to herein as the martensite transformation temperature (Mcurve).

In all cases the M curve decreases with increasing amounts of allelements. (See The Martensite Transformation Temperature in TitaniumBinary Alloys, by Pol Duwez, Trans. ASM, 45, p. 934 (1953).)

The precise location of the M temperature for each specific compositionwill depend upon several factors. Among these are the amount ofimpurities and state of equilibrium, both of which will vary undernormal conditions. The usual impurities will be: 0, N, H and C.Variations from the ideal state of equilibrium will also affect thestate of microstructure, as will the prior working. The rate ofquenching from above the M temperature will also cause a variation fromequilibrium conditions and thus affects the precise location of the Mtemperature. All these factors show that the precise M temperature ineach case is difficult to determine, but the specific temperature can beapproximated closely under each set of conditions, taking the effect ofthe above factors into consideration.

In FIG. 1, the p transus curve and the martensitic transformation (Mcurve are illustrated, using titaniummolybdenum binary alloys as typicaland for illustration purposes only. Above the beta-transus line, thealloy is in single phase and grain growth is very rapid, and the grainstructure of alloys quenched from this beta-phase field will haveextremely large grain size, e.g. 500 to 1000 In the alpha-beta field,above the martensitic transformation temperature, two phases exist,alphaphase plus beta-phase, and grain growth is more sluggish. Hotworking within this temperature range breaks up and refines the grains,and quenching therefrom, and then repeating the working and quenching,produces an ultrafine grain size. Some of the beta-phase during therapid quench in water to room temperature, is transformed intomartensite, but as ultrafine grains, which improves the desirablemechanical properties.

The following further examples are given as illustrative of the methodof the present invention and four typical alloys are given for thepurposes of illustration, but these typical examples are not intended tolimit the present invention to only these compositions.

Table I below lists the compositions of four illustrative compositions,identified as A, B, C and D. In Example 1 above, forging was one methodof hot working. In the following Examples 2, 3, 4 and 5 each of thesamples were processed by heating and hot-rolling in three successivesteps.

TABLE I.PROCESSING SEQUENCE OF 4 TYPICAL ALLOYS Alloy compositions Froman initial thickness of about 0.26 inch the final thickness of thespecimens, after the above sequence of Steps of reduction by hotrolling, was about 0.100 inch thick. Total reduction was about 62%. Therange of temperatures in the above table is :25 C. from the specifictemperature shown. The actual temperatures employed should preferably beas low as possible, within the permissible range, to minimizecontamination. Thus, in Example 2 above, where the nominal initialtemperature shown in Table II is 700 C., this approximates the Mtemperature. The subsequent temperatures to which the body is heated forsuccessive hot working is increased. Since M is diflicult to determine,as mentioned above, especially for the first Working step, it is betterto start the treatment at the lower end of the working range, and thenincrease the temperature for the subsequent treatments. However, it isimportant that at least the final hot-Working step be within the rangeof the martensitic transformation temperature and the beta-transustemperature.

In Examples 3 and 4, the hot working temperatures are lower than thosein the other examples because the M temperature drops with increasingamounts of alloying additives, as shown in FIG. 1.

The Compositions A, B, C and D in Table I were selected as typical forillustrative purposes for the following reasons, in addition to the factthat good results are demonstrated. Composition C is the same asComposition B, but with about 0.4 wt. percent silicon added forstrengthening the alloy by forming a dispersion Within the matrix, inaddition to the strengthening brought about by the ultrafine grain sizeachieved by the method steps of the invention.

(b) heating said body to a temperature below the alloys beta-transustemperature but above its martensitic transformation temperature,

(c) said hot-working of the heated alloy body being performed as itstemperature decreases, and

(d) quenching the hot-worked body.

2. A process according to claim 1, said alloy body consistingessentially of a titanium-base alloy.

3. A process according to claim 2, said titanium alloy having acomposition containing essentially about 6 Wt. pct. aluminum and about 4wt. pct. vanadium.

4. A process according to claim 2, said titanium alloy having acomposition containing essentially about 12 wt. pct. molybdenum.

5. A process according to claim 2, said titanium alloy having acomposition containing essentially about 18 wt. pct. molybdenum.

6. A process according to claim 2, said titanium alloy having acomposition containing essentially about 12 wt. pct. molybdenum andabout 4 wt. pct. silicon.

7. A process according to claim 1, at least the last of said cycles ofheating and hot-Working being performed above the martensitictransformation temperature and below the beta-transus temperature.

8. A process for producing superplastic alloy bodies having an ultrafinegrained microstructure, comprising the steps of plastically deformingsaid alloy body within a temperature range above the temperature atwhich martensitic transformation occurs but below its beta-transustemperature, quenching said alloy body, re-heating said alloy to saidtemperature range and again plastically deforming said body to produceat least one phase in ultrafine grain size form, and quenching saidalloy body.

9. A process according to claim 7, said plastic deforma- TABLEII.TYPICAL ROOM TEMPERATURE MECHANICAL PROPERTIES [Three-step hotrolling, plus annealing per Table I] Total 0.2% offset Tensile elonga-Reduction yield stress, strength, tion, of area, Ex. N0. Material p.s.i.p.s.i. percent percent 2 Pi-6M0 (812 C./10 73,000 85,200 37.0 62. 0

n. 3"... Ti-12Mo (732 O./10 88, 000 100, 500 40. 6 54. 9

n. 4 Ti-12Mo-0AS1 (732 105, 000 110, 000 28. 0 32. 0

C./10 min.)

TABLE III [Seven step cold rolling (46% reduction in thickness) plusanneal] Total 0.2% ofiset Tensile elonga- Reduction yield stress,strength, tion, of area, Ex. No. Material p.s.i. p.s.i. percent percent5 Ti-12Mo (732 C./10 109, 500 115, 400 13. 1 43 min.).

For comparison purposes, Table III shows mechanical properties an alloycorresponding to Composition B, but instead of the hot working accordingto the 3-step process of Example 3, a 7-step cold rolling to 46%reduction was used. It will be observed, from a comparison of Examples 3and 5, that the present invention results in a much higher ductility, asexpressed in percent elongation and percent reduction of area.

It will be obvious to those skilled in the art upon reading theforegoing disclosure that many modifications and alterations in themethod steps'and in the specific compositions many he made within thegeneral context of the invention, and that numerous modifications,alterations and additions may be made thereto within the true spirit andscope of the invention as set forth in the appended claims.

What is claimed is:

1. A process for producing ultrafine-grained alloy microstructures whichcomprises the steps of:

(a) subjecting an alloy body to at least two cycles of heating andhot-working which include tion being sufiicient to cause at least 20%reduction at each plastic deformation.

10. A process according to claim 8, said plastic deformation comprisinguniform reduction in thickness throughout said body.

References Cited UNITED STATES PATENTS L. DEWAYNE RUTTEDGE, PrimaryExaminer W. W. STALLARD, Assistant Examiner U.S. Cl. X.R. l48ll.5 F, 12

